Higher Energy Density Mediated Lithium-Sulfur Flow Batteries

There is a need for safe, reliable, high-capacity storage for long duration energy storage. The low cost and high specific capacity of sulfur make Li-S batteries ideal for this purpose. However, sulfur has poor electrical conductivity and Li-S batteries are prone to polysulfide shuttling that decreases the battery life. Additionally, lithium metal cannot be cycled at high rates or dendritic growth is produced. We have previously addressed the issues with the S by combining aspects of a static Li-S battery with aspects of a redox targeting system and flow battery. With this system we demonstrated that fundamental Li-S chemistry and novel SEI engineering strategies can be adapted to the hybrid redox flow battery architecture, obviating the need for ion-selective membranes or flowing carbon additives, and offering a potential pathway for inexpensive, scalable, and safe MWh scale Li-S energy storage. Here we present two key improvements to the system: (i) scaled up sulfur loading from 2.5 mgs cm -2 to over 50 mgs cm -2 (increasing energy density), and (ii) increased current density from 0.5 mA cm -2 to 10 mA cm -2 (increasing power density). Additionally, we are able to assemble the battery in the discharged state, improving the safety and decreasing manufacturing costs. To implement these improvements, we address the limitations due to the formation of dendrites on the Li metal anode. We first tested high surface area scaffolds in Swagelok cells to examine the effect of the increased effective surface area and seeding with lithiophilic materials like ZnO on Li metal deposition independent of the flow cell or S chemistry. Using a high effective surface area scaffold enabled increasing the applied current density from 0.5 mA cm -2 to 10 mA cm -2 , increasing the cycling rate by 20x. Furthermore, the addition of a lithiophilic seed layer decreases the nucleation overpotential and encourages uniform Li electrodeposition. A tailored flow field also increases the uniformity of Li deposition on the anode by enhancing the uniformity of the catholyte flow velocity. These modifications are first evaluated separately and then combined in a mediated Li-S flow battery where cycling rate and capacity retention are compared against a traditional planar Li anode and open flow field. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.